Architecture review
An outside read of a system you already run.
Freelance software architect · Stuttgart, Germany
Systems that have to hold, designed and built.
I am Daniel Hartmann. For twenty-five years I have been deciding where the boundaries in a system go and then writing the code on both sides of them — in public-sector platforms, SaaS products, and a multi-tenant booking platform I architect and operate myself. That last one is built day to day with AI coding agents, inside boundaries they cannot quietly cross — and it is in production.
- 25+ years, since 2001
- Public-sector platform across three German states
- A multi-tenant SaaS in production
- Built with AI coding agents, daily
- TypeScript · C# · Kotlin · Go · Kubernetes
Contract work is the default. The right permanent role — product ownership, a small senior team — is worth a conversation too: say which one you have.
Availability
- Status
- Available now
- Capacity
- Up to full-time
- Where
- Remote across the EU, or on site around Stuttgart
- Rate
- Day rate on request
- Contract
- Werk- or Dienstvertrag through octabits — DK2 Ventures UG (haftungsbeschränkt)
Services
Four ways this usually starts.
Each one has a shape, a duration and something written down at the end of it. Day rate on request; the first call costs nothing and sometimes ends with me saying no.
AI-assisted delivery, done carefully
Coding agents in a real codebase, with the guardrails that make it safe.
Greenfield system design
The shape decided before the team starts moving fast inside it.
Hands-on delivery
Building it, not just diagramming it.
Selected work
What it looked like in practice.
MUSES — school IT platform for school authorities
School IT staff in three federal states work in one interface instead of five vendor consoles, and no ticket is lost when the ticketing system is down.
- C#
- .NET 9
- Clean Architecture
- MediatR (CQRS)
- React 19
- Next.js
Reynt — multi-tenant rental & booking platform
Two architectural bets paid out in public: the payment providers (Stripe Connect and PayPal) were removed wholesale without touching settlement semantics, and four APIs moved from Elysia to Hono in a week with the domain packages untouched.
- TypeScript
- Bun
- Hono
- Nuxt 4
- Astro
- PostgreSQL (RLS)
Smartflow AI — workflow automation platform
Architecture and development of the backend for an AI-powered workflow automation platform.
- Kotlin
- KTOR
- TypeScript
- Python
- TensorFlow
- RabbitMQ
Current practice
I still ship every week — with agents at the keyboard.
Reynt is a production multi-tenant booking SaaS — thirteen packages, four APIs, three frontends — that I architected, wrote and operate myself, most of it with AI coding agents doing the typing. That is only defensible because of what sits underneath them: fitness functions that fail the build when a boundary is crossed, invariants placed where nothing can route around them, and integration tests against a real database in CI.
The agent is the cheap part. The guardrails are what I would actually be bringing to your codebase, and they are written down and checkable rather than asserted.
Reynt, by the parts that matter
- Packages
- 13
- APIs / frontends
- 4 / 3
- Tests
- Integration, real DB
- Tenant isolation
- Postgres RLS
- Hosting
- EU
In the open
Three projects you can read before you hire me.
Extracted from work that shipped, not written to be extracted. Documented, tested, and honest about their limits — the same code review you would get on your own codebase, except this one is public.
Library · TypeScript
octaflow
Durable workflows on the Postgres you already have.
Declare a DAG of Zod-typed steps. The engine runs each step as soon as its dependencies complete, persists every transition, retries failures, and picks up where it left off after a restart.
MITPre-1.0 · actively developed
Monorepo · TypeScript
@octabits-io/platform
The building blocks underneath everything else.
Two packages extracted from real products: an opinionated server framework toolkit and a Nuxt/Vue kit for admin SPAs. Everything heavy is an optional peer — you install only what you import.
MITPre-1.0 · published to npm
Kubernetes webhook · Go
cert-manager-webhook-porkbun
Wildcard certificates for Porkbun domains on Kubernetes.
A cert-manager ACME DNS-01 solver for domains hosted at Porkbun, which cert-manager has no built-in solver for. A maintained fork of an abandoned upstream — rewritten, hardened, and tested.
Apache-2.0Released · maintained
Approach
How I work, in 16 patterns you can check.
Each one names an established pattern, points at the shipped code that would break if I stopped doing it, and says what it costs. None of them belong to a language — the same decisions were made in C#, Kotlin, Rust and Go long before the TypeScript ones.
The shape of the code
Failure is a value, and the framework stays at the door.
How the rules survive
A rule you can break is not a rule.
Where it meets the world
Other people’s systems will be down. Plan for that, not around it.
What gets written down
Documentation that admits what the thing is bad at.
Writing
Notes from building the thing.
Got a system that needs an architect?
A paragraph about what you are building is enough to start. Mail is the fastest way through, and the reply comes from the person who would do the work.
Available now — up to full-time, remote across the EU, or on site around Stuttgart. Day rate on request.